The proposed studies are designed to examiine the mechanism of preferential inhibition of sodium reabsorption in juxtamedullary nephrons during extracellular volume expansion. These studies will involve micropuncture of the proximal superficial tubule and the bend of Henle's loop of juxtamedullary nephrons during water diuresis and after water diuresis plus benzolamide or 2% body weight Ringer loading. In addition, micropuncture of the late distal tubule and base of the papillary collecting duct will be performed under the same physiologic conditions. Another group of studies will examine the significance of net potassium addition along the straight proximal tubule, or descending limb of juxtamedullary nephrons. In these studies, animals will be chronically potassium loaded and papillary necrosis will be induced with bromoethylamine hydrobromide (BEA) 18-24 hours prior to study. The magnititude of net potassium addition between the late distal tubule and base of the papillary collecting duct will be compared in control and BEA treated rats.